Fuses are used in integrated circuits to customize their functions. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices.
One desired use of fuses is for circuit repair where the fuse activates redundant cells that take the place of a failed portion of the device. The fuse provides a simple and permanent way to change the circuit, and takes up very little area on the integrated circuit as compared to fully programmable approaches.
One approach for device repair would be to test the integrated circuit first to determine what portions of the device are not working. From this information the fuses that need to be blown in order to isolate the failed sections from use and activate appropriate replacement cells can be calculated. The fuses are then cut such as by using a programmable laser tool.
There are several requirements for designing, building and using fuses in integrated circuits. For example, the fuse construction must be compatible with processes being used to manufacture the integrated circuit. Also, additional processing to build the fuse must be minimized, because every additional step adds manufacturing costs and potential yield killing defects. The fuse must be able to be broken consistently with a high yield, and remain electrically open over the lifetime of the device. Similarly, unbroken fuses must maintain a reliable electrical connection over the life time of the device. Further, the process for opening the fuses must be reliable, inexpensive, and selective to the specific fuse to be opened.
These constraints tend to create a variety of problems in regard to the fabrication and use of fuses. For example, the ability to form consistent hole depths in the oxide above the fuse bank requires tighter process control to be placed on the etch and deposition steps. Additional steps are required to open the oxide above the fuse bank (Masking and Etch steps). The laser spot size sets the fuse to fuse spacing, window opening, and damage region, which tends to result in a large fuse structure. The suppression of oxide damage requires a crack arresting ring of interconnect metal surrounding the fuse bank. The large opening and damage region in the fuse bank provides an entry point for impurity diffusion into the circuit area below, which can degrade the reliability of the part.
With current methods it is difficult to meet all of these requirements at the same time. As a result, fuse integrations tend to require many compromises with respect to yield, additional processing steps, and cost. This significantly limits the usefulness of fuses to control integrated circuits.
FIG. 1 depicts a typical aluminum fuse bank structure 11, where some fuses 13 are blown using a laser pulse, and other fuses 15 are left intact. FIG. 2 depicts a cross sectional diagram of an aluminum laser fuse structure such as used in a 130 nanometer copper damascene technology to activate redundant memory cells.
Fabrication of the fuse structure depicted in FIG. 2 requires an additional masking and etch step, and a more complicated stack of passivation layers. In order to avoid additional metal deposition and masking steps, the aluminum fuse must be made from the same aluminum material and thickness as the bonding pad. As a result, the fuse yield is low, which limits the number of fuses that can be used on an integrated circuit before the yield losses become unacceptable. The long term reliability of the aluminum fuse is also an issue. It is well known that aluminum materials migrate under physical or electrical stress. Fuses have been known to reconnect over time if the fuse gap is small.
Another issue with the laser blown aluminum fuse is that the yield and reliability are dependent on many factors that can vary during manufacturing. This includes the thickness and width of the aluminum fuse link, the thickness of the oxide remaining over the fuse, and the laser power that is applied to open the fuse.
A further drawback of the aluminum fuse is that it takes a relatively considerable amount of energy to break it. As a result, the passivation layer above the fuses tends to be damaged, which can affect the reliability of neighboring fuses that are not intended to be broken. In addition, any electrical interconnect or transistor in the area below the broken fuse can be damaged by the breaking process, which means that area cannot be used for other electronic elements, and becomes wasted space in the integrated circuit design.
Thus, for some of the reasons described above, current electrical fuse technology is not widely accepted as a means of circuit or memory repair. One concern with the use of fuse technology for circuit repair is the stability of the fuse structure in regard to varying temperature, electrical, and radiation conditions over time. Another concern is the generally-unknown length of time that the fuse structure can sustain the open or closed (0 or 1) circuit setting with which it is programmed.
Another detriment of aluminum fuse architecture is the fact that the blown fuse is not passivated after it is blown and the circuit may become reconnected due to environmental conditions (moisture or metals in the packaging material) forming the connection between the severed links. Also, under high tensile stress conditions aluminum metal migration can take place and reconnect the blown fuse links.
In FIG. 1, showing an aluminum fuse bank, an aluminum metal guard ring surrounds the fuse bank. The guard ring arrests cracking and confines damage caused during the laser trimming of the aluminum fuse. This guard ring takes up additional area, and so to minimize this effect the fuses are all contained in the same generalized area.
These drawbacks of the existing fuse processes are addressed by generally limiting the number of fuses that are used in an integrated circuit, or by using programmable circuits, which take up more space on the integrated circuit, but tend to be a more robust technology. Further, the use of programmable circuits adds considerably to design and manufacturing costs.
What is needed, therefore, is a system for programming integrated circuits that overcomes problems such as those described above, at least in part.